| 1. |
- Alfredsson, Viveka, et al.
(författare)
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In-situ studies of the formation mechanism of SBA-15
- 2005
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Ingår i: Nanoporous Materials IV, 156. - 0444517480 ; 156, s. 69-74
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Konferensbidrag (refereegranskat)abstract
- The formation process of SBA-15 has been investigated by time resolved TEM, in-situ SAXS/SAXD and in-situ NMR. The synthesis was done at 35 degrees in acidic solution with tetramethylorthosilicate as the silica source and Pluronic P123 as the structure directing amphiphile. The initially formed silica-polymer composite consists of flocs of spherical micelles in a silica matrix. The micelles then coalesce forming rods that eventually pack into the ordered array of which SBA-15 consists.
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| 2. |
- Flodström, Katarina, et al.
(författare)
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Formation of a new Ia(3)over-bard cubic meso-structured silica via triblock copolymer-assisted synthesis
- 2003
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 125:15, s. 4402-4403
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Tidskriftsartikel (refereegranskat)abstract
- A new mesoporous silica structure with cubic Iad symmetry has been synthesized. The structure has very large unit cell dimensions, up to 250 Å in the as-synthesized form and 222 Å after calcination, and the surface area is around 700 m2/g. The syntheses were done according to well-established synthesis conditions with the triblock copolymers Pluronics P103 or P123, except for the addition of NaI to the synthesis mixture. Small-angle X-ray scattering revealed that the sample has Iad symmetry. According to electron micrographs, the structure is similar to that of MCM-48, and we thus propose that the structure lies on the gyroid minimal surface.
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| 3. |
- Flodström, Katarina, et al.
(författare)
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In situ synchrotron small-angle X-ray scattering/X-ray diffraction study of the formation of SBA-15 mesoporous silica
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:12, s. 4885-4891
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Tidskriftsartikel (refereegranskat)abstract
- The initial stages of the formation of SBA-15 have been studied by in situ SAXS/XRD using synchrotron radiation. Modeling of both the diffuse scattering and the X-ray diffraction patterns obtained at different stages of the reaction results in a detailed description of the different reaction steps. The first step in the formation is the liquid-liquid phase separation of spherical P123-silicate hybrid micelles after which nucleation and growth of the 2D hexagonal phase occurs. Two-dimensional electron density maps calculated on the basis of the intensities of the Bragg reflections suggest that changes in the degree of intermicellar condensation are responsible for the time-dependent observations. The silica source (alkoxysilanes) may be partially unhydrolyzed long after formation of the hexagonal structure, since the kinetics of the mesophase evolution is notably slower when TEOS is used as the silica precursor as compared to TMOS. The results obtained for SBA-15 are compared with other published data on the formation of SBA-15 and of the smaller pore MCM-41 materials.
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| 4. |
- Flodström, Katarina, et al.
(författare)
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Influence of the block length of triblock copolymers on th formation of mesoporous silica
- 2003
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Ingår i: Microporous and Mesoporous Materials. - 1387-1811. ; 59:2-3, s. 167-176
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Tidskriftsartikel (refereegranskat)abstract
- The effect of different block lengths of Pluronic surfactants, (EO)(x)-(PO)(y)-(EO)(x), in the formation of mesoporous silica has been investigated. The syntheses were performed in micellar solution of the surfactant under acidic conditions. The materials were characterized by SAXS, TEM and nitrogen adsorption measurements. The EO-block length of the polymers determines the mesostructure of the silica. For the hexagonal material (SBA-15) the wall thickness is largely dependent on the length of the EO-blocks, while the PO-block length has a great effect on the pore diameter. Furthermore, the PO-block length influences the templating ability, as longer PO-blocks result in more highly ordered domai s and well defined particles. The synthesis temperature also influences these parameters. (C) 2003 Elsevier Science Inc. All rights reserved.
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| 5. |
- Flodström, Katarina, et al.
(författare)
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Mechanism of mesoporous silica formation. A time-resolved NMR and TEM study of silica-block copolymer aggregation
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:3, s. 680-688
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of the synthesis of a mesoporous silica material SBA-15 is followed using time-resolved in situ H-1 NMR and transmission electron microscopy (TEM). Block copolymer-silica particles of two-dimensional hexagonal symmetry evolve from an initially micellar solution. The synthesis was carried out with the block copolymer Pluronic P123 (EO20-PO70-EO20) at 35 degreesC and using tetramethyl orthosilicate as the silica precursor. By using TEM, we can image different stages during the evolution of the synthesis. Floes of spherical micelles held together by the polymerizing silica are observed prior to precipitation. With time, the structure of these floes evolves and the transition from spherical to cylindrical hexagonally packed micelles can be monitored. The signal from the methyl protons of the PO part was recorded with H-1 NMR. One observes a continuous increase in the signal width but with distinct changes in the spectral characteristics occurring in narrow time intervals. The spectral changes can be attributed to structural changes of the self-assembled aggregates. The H-1 NMR and TEM studies reveal the same mechanism of formation. It is concluded that the aggregation is caused by a micelle-micelle attraction induced by oligomeric/polymeric silica that adsorbs to the EO palisade layer of the micelles and has the ability to bridge to another micelle. This adsorption also favors the formation of cylindrical aggregates relative to spherical micelles. The sequence of NMR and TEM observations can then be interpreted as the following sequence of events: (i) silicate adsorption on globular micelles possibly accompanied with some aggregate growth, (ii) the association of these globular micelles into floes, (iii) the precipitation of these floes, and (iv) micelle-micelle coalescence generating (semi)infinite cylinders that form the two-dimensional hexagonal packing.
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| 6. |
- Flodström, Katarina
(författare)
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Mesoporous Silica Formed with Triblock Copolymers
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The formation of highly ordered mesoporous silica materials, using PEO-PPO-PEO amphiphilic triblock copolymers (so-called Pluronics) as structure directing agents, has been studied. The materials are formed through the addition of a silica source to a polymer solution of low concentration (2.5 wt ) under acidic conditions. The products have BET surface areas of approximately 800 m2/g. The unit cell parameter is around 100 for hexagonally structured materials, and in the range of 150-250 for cubic materials. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) have been the main characterisation techniques. Pluronics with different lengths of the PEO- and PPO-blocks were used in the syntheses. It was shown that different block lengths influence the structure formed, as well as the pore size and wall thickness of the materials. Materials with hexagonal (p6mm), micellar cubic (Im3m) and lamellar structures were obtained using Pluronics with different PEO/PPO compositions. Temperature has also shown to be an important synthesis parameter, as the conformation of the polymers, as well as the silica polymerisation, is highly temperature dependent. Further, the effect of the addition of salt to the synthesis mixture has been studied. Through the addition of sodium iodide, a new large-pore bicontinuous cubic silica (Ia3d) was synthesised. The mechanism of formation for the materials has been studied in detail. In situ synchrotron SAXS, in situ time-resolved 1H NMR and time-resolved TEM have been used to follow the time evolution of the syntheses. The different structures are formed via different mechanisms, illustrating the importance of understanding the mechanism in order to influence and control the synthesis to obtain the desired product.
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| 7. |
- Flodström, Katarina, et al.
(författare)
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Time-resolved in situ studies of the formation of cubic mesoporous silica formed with triblock copolymers
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:23, s. 10311-10316
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism of formation of two different cubic mesoporous silica materials formed with Pluronic triblock copolymers is investigated with in situ time-resolved small-angle synchrotron X-ray scattering, in situ time-resolved H-1 nuclear magnetic resonance, and time-resolved transmission electron microscopy. The materials studied are the micellar cubic (Im (3) over barm) SBA-16 formed with Pluronic F108 and the bicontinuous cubic (Ia (3) over bard) silica material formed with Pluronic P103 and NaI. The formation mechanisms of the two cubic structures are shown to be dissimilar. For the Im (3) over barm material, in the early stages of the synthesis, floes of unordered micelles are observed, but areas where the micelles have started to order are also present. With time, there is an increase in order; however, there is a coexistence of unordered micelles and ordered material all through this study. The bicontinuous cubic silica is formed via a different path. The system is phase-separated already before the addition of the silica source, which implies that a concentrated phase is present, acting as the structure director of the Ia (3) over bard structure. The results are compared with earlier reports on the formation of the hexagonal SBA-15 material.
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| 8. |
- Kipkemboi, Pius, et al.
(författare)
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Triblock copolymers as templates in mesoporous silica formation : Structural dependence on polymer chain length and synthesis temperature
- 2001
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 17:17, s. 5398-5402
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Tidskriftsartikel (refereegranskat)abstract
- Mesoporous silica can be formed with amphiphilic molecules acting as structure-directing templates in dilute systems. The templating properties of five related triblock copolymers, (EO)x-(PO)y-(EO)x (Pluronic L101, P103, P104, P105, and F108), have been investigated regarding the length of the EO blocks as well as the temperature of the synthesis. It was found that the number of EO units is essential in determining which silica mesophase is obtained. Lamellar structure is obtained with short EO chains (4 units); hexagonal structure, with medium length EO chains (17-37 units); and cubic structure, with long EO chains (132 units). The hydrophobicity of the (EO)x-(PO)y-(EO)x polymer is strongly affected by the temperature. This is to some extent reflected in the silica mesostructure formed; a multilamellar vesicle morphology can be obtained at elevated synthesis temperatures. Also, the temperature has an influence on particle size.
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| 9. |
- Xu, Changming, et al.
(författare)
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Low temperature densification of B4C ceramics with CaF2/Y2O3 additives
- 2012
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Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 35, s. 311-314
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Tidskriftsartikel (refereegranskat)abstract
- This study deals with low temperature densification of boron carbide (B4C) ceramics with combined employment of CaF2/Y2O3 sintering additive and Spark Plasma Sintering (SPS) technique. It demonstrates that a small amount addition of CaF2/Y2O3 has strong effect on the densification of B4C ceramics. With 0.5-1 wt.% CaF2/Y2O3 addition, nearly full-densification up to 99.1% of B4C ceramics can be attained at a temperature as low as 1700-1750 degrees C.
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| 10. |
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| 11. |
- Xu, Changming, et al.
(författare)
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Spark plasma sintering of B4C ceramics : The effects of milling medium and TiB2 addition
- 2012
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Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 30:1, s. 139-144
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Tidskriftsartikel (refereegranskat)abstract
- Boron carbide (B4C) ceramics, with a relative density up to 98.4% and limited grain growth, were prepared at 1600-1800 degrees C by spark plasma sintering (SPS) technique. The effects of powder milling medium (water and 2-propanol) on the powders' surface characteristics and TiB2 addition on the sintering densification were investigated. The ball milling processing of B4C powders in water can promote the sintering of B4C ceramics. A B2O3 layer on B4C particle surface is concluded to promote the densification of the B4C ceramics at an early sintering stage. This B2O3 layer, which normally inhibits the densification process at the final stage of the sintering, can be reduced through reaction with TiB2 particles, resulting in further densification of the B4C ceramics.
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